1. Field of the Invention
The present invention relates to a motor apparatus with a voltage step-up function including a motor and an inverter.
2. Description of Related Art
Such a motor apparatus with a voltage step-up function is known, for example, from Japanese Patent Application Laid-open No. 2008-306914. This motor apparatus includes a capacitor, a three-phase inverter, a three-phase motor and a battery. The three-phase inverter includes DC ends and AC ends. The DC ends of the three-phase inverter are connected to the capacitor. The three-phase motor includes a star-connected three-phase winding. The respective phase ends of the three-phase winding are connected to the AC ends of the three-phase inverter. The positive and negative terminals of the battery are connected between one of the DC ends of the three-phase inverter and the neutral point of the three-phase winding. The three-phase inverter is controlled so that AC phase currents each containing a DC component flow through the respective phase windings. As a result, the DC voltage of the battery is stepped up, and supplied to the capacitor, while the three-phase motor generates torque depending on the voltage supplied from the capacitor.
Incidentally, the torque instantaneous value T of the three-phase motor is determined by the induced voltages and the phase currents of its respective phases as shown by the equation 1.T=k·(Eu·iu+Ev·iv+Ew·iw)
where Eu, Ev and Ew are the instantaneous values of the induced phase voltages, iu, iv and iw are the instantaneous values of the phase currents, and k is a constant vale concerned about torque.
The induced voltage of each phase contains a third-order harmonic component other than its fundamental component. Also, each phase current contains a DC component. Accordingly, the torque instantaneous value T of the three-phase motor contains a torque ripple depending on the third-order harmonic component of each induced voltage and the DC component of each phase current.T∝(Eu0+E3rd)·(iu0+i0)+(Ev0+E3rd)·(iv0+i0) +(Ew0+E3rd)·(iw0+i0) =Eu0·iu0+Ev0·iv0+Ew0·iv0+E3rd·(iu0+iv0+iw0) +i0·(Eu0 +Ev0+Ew0)+3·E3rd·i0=Eu0·iu0+Ev0·iv0Ew0·iw0+3·E3rd·i0(∵iu0+iv0+iw0=0, Eu0+Ev0+Ew0=0)
where Eu0, Ev0 and Ew0 are the basis components of the induced phase voltages, iu0, iv0 and iw0 are the basic components of the phase currents, E3rd is a third order component of the induced voltages, and i0 is an offset component of the phase currents.
Japanese Patent Application Laid-open No. 2000-184633 discloses a brushless DC motor as a motor capable of reducing its torque ripple. This brushless DC motor includes a stator core in which a part of the front end of each of its tooth portions is cut, so that concentration of magnetic flux is lessened to reduce the torque ripple.
Incidentally, the technique to reduce the torque ripple by cutting a part of the front end of each of the tooth portions of a stator core is effective only when respective phase currents are symmetric to one another. Accordingly, the torque ripple increases to the contrary when this technique is applied to a motor in which each of its phase currents contains a DC component. Hence, there is a technical challenge that the torque ripple cannot be reduced for a motor in which AC currents each containing a DC component are passed to its phase windings as phase currents.